The present invention relates generally to gold alloy compositions, and, more particularly, to improved 18-karat green gold alloy compositions.
Traditionally, 18-karat green gold alloys have been used to provide a color contrast to other alloys in multicolored jewelry articles, such as bracelets, necklaces and the like. Most existing 18-karat green gold alloys are based on a familiar gold-silver-copper system, and typically contain high concentrations of silver. These green golds are fairly soft and cannot be appreciably age-hardened.
The color of gold alloy compositions is no longer a matter of subjective impression. Rather, color is now determined objectively in terms of its component colors a (green-red) and b* (blue-yellow) on a CieLab color-measuring system. See, e.g., G. Raykhtsaum et al., xe2x80x9cThe Color of Goldxe2x80x9d, A. J. M. (October 1994), the aggregate disclosure of which is hereby incorporated by reference.
Moreover, some gold alloys have been developed that offer the capability of reversible hardness, by selective application of an appropriate heat treatment, between their annealed-hardness and aged-hardness values. In many cases, there is a considerable disparity between these hardnesses. Hence, an alloy may be annealed to lower its hardness value. This allows the alloy to be worked more easily. After the alloy has been worked, as by forming an article of jewelry therefrom, the item may be aged-hardened to a higher hardness value to increase its resistance to denting and deformation. However, if there is a subsequent need to rework or repair the item, it may be annealed to reduce its hardness to its annealed-hardness value. After the item has been reworked or repaired, it may be age-hardened to a higher hardness value. Other gold alloys having this xe2x80x9creversiblexe2x80x9d hardness feature are shown and described in U.S. Pat. No. 5,180,551, the aggregate disclosure of which is hereby incorporated by reference.
For example, one alloy, denominated as Alloy 1 herein, contains about 75.0% gold, about 22.5% silver, and about 2.5% copper. This alloy has an annealed hardness of about 100 Vickers Hardness Number (xe2x80x9cVHNxe2x80x9d). While this has an acceptable green color, it cannot be age-hardened to a substantially greater value.
In an attempt to increase the hardness, the concentration of silver has been lowered. However, as the silver content decreases, these alloys have been found to lose there greenish tint. For example, another known alloy, designated Alloy 2 herein, contains about 75% gold, about 16% silver, and about 9% copper. This alloy was found to have an annealed hardness of about 130 VHN, and was capable of being age-hardened to about 170 VHN. However, the harden-ability of this alloy was at the expense of its color. Rather than being a green, this alloy had a pale-yellow color.
Further decreases in the silver concentration has led to a pronounced color shift from green to yellow. For example, a third prior art alloy, designated Alloy 3 herein, contains about 75.0% gold, about 12.5% silver, and about 12.5% copper. This alloy was found to have an annealed hardness of about 150 VHN, and was capable of being age-hardened to about 250 VHN. However, the color of this alloy was found to be yellow.
None of these known prior art alloys is believed to: (1) have an attractive green color, and (2) be capable of reversible hardening between its annealed hardness value and its aged-hardness value.
Accordingly, it would be generally desirable to provide improved 18-karat green gold alloy compositions that retain their desirable green gold color, and are capable of being selectively hardened and softened, at will, between their annealed and aged-hardness values.
The present invention provides various improved 18karat green gold alloy compositions that are capable of being reversibly hardened and softened between their annealed and aged-hardness values.
The improved alloy compositions broadly comprise: about 75.0% gold; about 6.0-7.0% silver; about 9.0-11.7% copper being age-hardened to a hardness of at least about 240 VHN by being heated to about 550 xc2x0 F. for about one hour, and thereafter being permitted to cool in air.
The improved compositions may further comprise about 0.3% cobalt, about 0.005% iridium and/or about 0.5-1.9% indium. A first preferred embodiment contains about 75.0% gold, about 7.0% silver, about 10.7% copper, about 7.0% zinc, and about 0.3% cobalt. A second preferred embodiment contains about 75.0% gold, about 7.0% silver, about 10.2% copper, about 7.0% zinc, about 0.3% cobalt, and about 0.5% indium. The annealed condition of the alloys obtained by heating the composition to about 1150xc2x0 F. for about thirty minutes, followed by quenching in water. The age-hardening condition is obtained by heating the composition to about 550xc2x0 F. for about one hour, after which the composition is permitted to cool in air.
With the improved alloys, the aged-hardness is about 1.33-2.0 times greater than the annealed hardness obtained by these conditions. Moreover, the hardness of the alloys is reversible between its annealed and aged-hardness values. This means that an annealed alloy may be selectively age-hardened to a greater hardness value by heating the alloy to about 550xc2x0 F. for about one hour, and by thereafter allowing the composition to cool in air. Conversely, an age-hardened alloy may be softened by heating the alloy to about 1150xc2x0 F. for about thirty minutes, followed by a water quench. Thus, the hardness of the alloys is controllably reversible between the annealed and aged-hardness values. This is highly desirable because it allows the alloy to be worked or formed in its softer annealed state. Thereafter, a particular article may be age-hardened to a greater hardness value. If the item is ever in need of repair, its hardness may be reduced by heating the article to about 1150xc2x0 F. for about thirty minutes, and by quenching in water. After being reworked or repaired, the article may be again age-hardened to a greater hardness value by heating the alloy to about 550xc2x0 F. for about one hour, and by allowing the piece to cool in air. Hence, the hardness of the alloy is selectively reversible. Importantly, the improved alloys have been found to exhibit a desirable green color, which ranges between about 1.5 to about 3.0 CieLab a* units, and between about 19 to about 26 CieLab b* units.
These and other objects and advantages will become apparent from the foregoing and ongoing written specification, and the appended claims.